Image sensing devices which operate by performing the steps of sensing incident optical radiation, converting the radiation into charge carriers, and storing the charge carriers in photosensitive material are well known in the art.
Most conventional image sensing devices are based on charge-coupled device (CCD) technology. Fabricating a CCD-based imager requires a number of specialized manufacturing operations Moreover, a CCD-based imager utilizes nonstandard voltages, which necessitates a complex power supply and associated distribution circuitry. These requirements make more difficult the task of integrating circuits employing CCDs into an electronic system and increase the complexity of the fabrication process.
Certain design and fabrication difficulties associated with CCD-based image sensing devices can be overcome with an imager comprising pixels with MOSFETs and fabricated using a CMOS process. However, voltage threshold variation among MOSFET devices makes it difficult to achieve uniform and precise resetting of pixels, and produces fixed-pattern noise in the output signal. What is needed is a MOSFET-based image sensing device in which fixed-pattern noise attributable to pixel-to-pixel voltage threshold variation is effectively eliminated. Accordingly, it is an object of the present invention to provide an imaging device which can be fabricated using a CMOS process and which yields a signal with low levels of fixed-pattern noise.
It is a further object of the invention to provide such an imaging device which incorporates a resetting operation that is minimally affected by pixel-to-pixel variation in operating parameters.